Inspection is an important aspect of the fabrication process for integrated circuits. Inspection is generally used to detect imperfections in the integrated circuits, such as pattern defects, particles, or other foreign matter.
However, it is becoming ever more difficult to detect small defects on rough surfaces utilizing a dark-field scanning surface inspection system; more specifically, particles and contaminants in the range of sixty nanometers to two hundred nanometers polystyrene latex sphere (PSL) equivalent size on films used in integrated circuit processing. These films can be, for instance, polysilicon, low-k dielectrics, silicon-on-insulator, strained-silicon-on-insulator, and other similarly rough materials that are at least partially transparent to optical wavelengths.
Current methods on smooth surfaces, such as bare silicon, use oblique illumination with a polarization combination called “P-U” which indicates that the incident light is polarized (e.g. in the plane of incidence), and the collected scattered light is unpolarized, meaning that all polarizations are collected. This method achieves the greatest sensitivity on very smooth surfaces. However, rough surfaces scatter too much light in P-U, and the scattering from small defects is swamped by the surface scattering. In these cases, it is well known that the “double-dark-field” configuration is useful for inspecting rough surfaces for contamination and defects.
In particular, using s polarized obliquely incident light, e.g. polarized perpendicular to the plane of incidence, results in a dark fringe at the surface, and very little light is scattered from the surface itself. In conjunction with an analyzer oriented perpendicular to the plane of scatter, and an aperture limited to “side-angle collection,” the unwanted surface background contribution can be reduced by several orders of magnitude. Large particles and defects resting on the surface can then be detected relatively easily, as they do not experience the dark fringe, and therefore they perturb (scatter) the incident electric field efficiently compared to the surface.
Side-angle collection means that the collected scattered light is limited to azimuthal angles reasonably close to plus or minus ninety degrees with respect to the plane of incidence. For example, in some instruments there are two configurations for side angle collection: one which collects light within ten degrees of plus or minus ninety degrees (twenty degrees azimuthal width on the plane of incidence) and one which collects light within twenty degrees of plus or minus ninety degrees (forty degrees azimuthal width).
These methods of S-S polarization with side-angle collection work well for particles having a size greater than approximately one-half the wavelength of the incident light. The S-S side-angle configuration is very effective at reducing the scattering from the surface. Unfortunately, it is also very effective at reducing the scattering from small defects, where small is defined as a diameter that is less than about forty percent of the wavelength of the incident light. Once the defect size increases to approximately forty percent of the wavelength of light, a typical defect begins to scatter significantly into the side-angle collector. Since the surface scattering is suppressed, this provides a significant signal-to-noise advantage.
In the past, integrated circuit fabrication processes using rough surfaces were generally subject to failure by defects having a size of approximately two hundred nanometers and larger. Such defects could be detected using an illumination wavelength of 488 nanometers, which was used at the time. However, now there is a need to detect defects having a size of less than one hundred nanometers on rough, partially transparent surfaces. Even an ultra violet wavelength of 355 nanometers is not sufficient to efficiently detect such defects when combined with the S-S side-angle technique.
Electromagnetic wave theory allows one to self-consistently calculate the electric field at every position within or above a multi-layer film structure, given the thicknesses and refractive indices of the layers. In general, fields scattered from an object are proportional to the magnitude of the incident field in the vicinity of the object. Therefore, if the incident electric field magnitude is adjusted to be relatively small at a point on the rough film surface, and substantially larger at a point that is tens of nanometers above the surface, where many defects and contaminants reside, improved defect scatter signal to rough surface scatter signal may be observed in an inspection system.
The S-S side-angle collection strategy, described above, utilizes this in part, but presumes that the rough film is opaque, and that there is very little light that is reflected from the substrate or other layer interfaces underneath the rough film. In this case, the potential for enhanced defect scattering relative to surface scattering is limited, because with only two waves interfering, namely the incoming wave and the wave reflected from the rough surface, significant changes in field strength are not possible over distances much smaller than one-half the wavelength of light.
On the other hand, if additional film layers exist between the uppermost rough film layer and the substrate, there are multiple reflected waves, which can interfere to produce dramatic differences in electric field strength near the surface of the rough film, over distances of one-tenth the wavelength of light, or smaller.
Many rough films, particularly metals, are relatively opaque at visible and ultraviolet wavelengths. However, other rough films of significance, such as polysilicon, can be relatively transparent. Also, with advanced integrated circuit processing, films can be significantly thinner than inspection wavelengths. Light can easily penetrate a polysilicon film to interact with the substrate and other films, and reflected light from these other interfaces can again traverse the polysilicon film, with only modest attenuation, to interfere at the rough surface.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.